This invention relates to a liquid fuel fired melting furnace for particulate material and more particularly it relates to a furnace apparatus with a shaft preheater section mounted above a melting chamber wherein the surface of a lower portion of a free-standing pile of preheated feedstock is melted.
The initial objective of this invention disclosed herein was to solve various problems relating to the melting of blast furnace slag and other siliceous raw materials for the production of mineral wool insulation. Normally the melting process was carried out in a cupola or shaft type furnace into the top of which the raw material was fed along with a solid fuel, such as a metallurgical coke. The fuel was ignited and combustion air was circulated upwardly from the bottom of the furnace through the mixture of raw material and fuel. This caused the fuel to be consumed, heat to be produced and the raw material to be melted. The melted material gravitated to the bottom of the furnace where it was drawn off and formed into mineral wool. Additional raw material and fuel were fed intermittently or continuously into the top of furnace to replenish the portion of the mixture which was consumed or melted.
A recent problem with the process is that the cost of the high quality coke required to produce a suitable product has risen significantly. Another problem which has plagued prior art processes using solid carbonaceous fuel is that the intimate contact between the melted material and the unburned fuel, particularly at the high temperatures involved, resulted in contamination of the product by the fuel. At times the flue gases which were emitted from the furnace contained contaminants, especially if a sulfur containing solid fuel were used, and thus had to be treated by anti pollution devices before they could be discharged into the atmosphere. Another critical problem was the lack of operational controls. After an effective particle size had been selected along with the proper ratio of fuel in the charge, the only remaining controllable variable in the process was the flow rate of combustion air through the furnace. All of these factors added to the cost of production or difficulty of operation.
Natural gas appeared to be an ideal alternative fuel because it is a relatively clean burning fuel and thus will not contaminate the feedstock material or create an air pollution problem if it is used properly. Also in most instances it is more economical to use than other sources of heat. However, when slag or a similar particulate material was melted in a shaft furnace by itself, rather than in a mixture containing a solid fuel, it tended to coalesce randomly in the shaft thereby impeding the uniform upward flow of hot combustion gases through the load and also impeding the uniform downward flow of the feedstock.
Accordingly it is a general object of this invention to solve the aforementioned problems and to provide an improved liquid fuel fired furnace apparatus and related method for economically melting flowable particulate material. It is another object to provide such a furnace and method which have more controllable variables and thus are operationally more flexible. It is still another object of this invention to provide a melting furnace with a gas fired heating system which effectively distributes and efficiently utilizes the products of combustion to preheat and melt particulate feedstock, such as blast furnace slag.